Compressors, in particular refrigerator compressors, are usually driven by conventional rotary electric motors and a crank mechanism. Resulting high side forces on the compressor piston require oil lubrication of the piston-cylinder interface. Thus, the refrigerant must be compatible with oil and there is appreciable power loss from friction in the mechanism. In the search for refrigerants to replace ozone depleting CFCs, oil compatibility is a substantial restriction.
Friction losses in the conventional crank mechanism waste energy. It is therefore advantageous to drive the compressor piston with a linear motion motor, which eliminates crank mechanisms and reduces side forces on the piston to a very low value, thereby eliminating the need for oil and making possible the use of gas bearings for the piston cylinder interface. Gas bearings have very low frictional power loss and practically no wear. The advent of high efficiency permanent magnet linear motors, such as the design disclosed in U.S. Pat. No. 4,602,174, makes the replacement of rotary motors by linear motors in a compressor economically feasible. However, such replacement poses a problem because if it is done, the rigid restraint on piston motion imposed by a crank mechanism no longer exists. The linearly reciprocating device has no inherent limits except collision of the reciprocating part with a stationary part.
A compressor piston driven by a linear motor will take up an average position that depends on the gas forces acting on the piston, and will reciprocate around the average position. As gas forces change, both the average component of position and the alternating component of position may change. Without some means of detecting the piston position and using the detected position in a feedback loop that controls the voltage applied to the motor, it is possible for the piston to hit the cylinder head, thus generating objectionable noise and possibly damaging the compressor. Another compelling reason for measuring piston position is that such measurement can be used to control the flow rate of mass pumped through the compressor in response to changing demands. In a refrigerator compressor, control of flow rate in response to changing ambient temperature can significantly improve the thermodynamic efficiency of the refrigeration cycle.
For purposes of preventing piston-cylinder head collisions and controlling mass flow rate through the compressor, one particular piston location is especially significant, namely the piston's location at its closest approach to the cylinder head. This special location can be determined by many types of position sensors, for example, optical detectors or proximity sensors based on eddy current generation. Use of such sensors would add to cost, could degrade reliability, and would create significant installation problems, particularly the need to bring several wires out through the wall of a pressure vessel in the case of refrigerator compressors.
The present invention is a method of measuring piston position at closest approach to the cylinder head without such an added sensor. It uses measurements of motor voltage and current made outside the compressor, as inputs to a digital or analog computation device to determine the piston position on closest approach based on known linear motor properties and known dynamics of piston motion.